The present invention relates to a radiant beam exposure apparatus of a raster scan system and, more particularly, a method for compressing pattern data and a pattern data compression processing circuit, for the radiant beam exposure apparatus.
The prior art and its defects will first be briefed. The description will proceed by using an electron beam; however, it is applicable for charged beams such as X-rays. An electron beam exposure system is generally classified into a one-stroke type and a projection type. The one-stroke type is further divided into a vector scanning system and a raster scan system. In the raster scan system, a pattern is divided in a mesh-like fashion, and black meshes are each represented by "1" and white meshes each by "0". To depict a part of an IC pellet 250 .mu.m .times. 5000 .mu.m with the mesh pitch 0.5.mu.m, the picture elements are EQU 250/0.5 .times. 5000/0.5 = 500 .times. 10.sup.4 = 5 .times. 10.sup.6.
Accordingly, the memory must have at least 5M bits memory capacity for storing the picture elements of 5 .times. 10.sup.6. The memory cycle of the present minicomputer is at most 800 nano sec. For the parallel processing of 16 bits, 50 nano sec/bit is the upper limit of the data transfer rate. Incidentally, the data transfer rate corresponds to 20M bits/sec.
With progress of the IC technology, the electron beam exposure system with little diffraction has attracted attention. Nevertheless, it has been not yet put to practical use in the industrial field. The most important reason for this is that the length of the exposure time is too long. For example, when an IC pattern is depicted on a wafer of 10 cm.sup.2 by using an electron beam of 0.5 .mu.m, it takes 2000 seconds under a condition that the data transfer time is 50 nano sec/bit and the time for overhead is negligible. To practically use the electron beam exposure apparatus in the industrial field, the exposure time must be below about 300 seconds.
In the light of development of electronic computers of late years, it is very difficult to improve the pattern depicting speed by about one digit in several years for large capacitive memory 1 to 10M bits, particularly from a cost point of view.